Technical Field
The present invention relates to a variable capacitance device and an antenna device that utilizes the variable capacitance device.
Background Art
In NFC (near field communication) modules used for mobile FeliCa, a phenomenon has been known to occur in which reception sensitivity decreases as a result of the resonant frequency shifting away from 13.56 MHz due to variations in the antenna coil, for example. To correct these shifts in the resonant frequency, frequency adjustment circuits that include capacitors are placed inside the modules, all parts are inspected before the modules are shipped, and the capacitance of the capacitors is minutely adjusted.
Conventionally, switched capacitors, in which FET (field effect transistor) switches are connected in series in a fixed capacitance element, have been used. Switchover settings are then written onto a control IC (integrated circuit) during the pre-shipping inspection, and when NFC is being used, the module switches over to the FET mode and minutely adjusts the capacitance of the capacitor.
However, general-purpose ceramic capacitors, which have become cheaper than FET switches in recent years and which can handle high voltages, have been considered as a possible replacement for FET switches. Ceramic capacitor materials have a property that has been actively utilized: the capacitance decreases as a DC bias voltage is applied.
The capacitance of ceramic capacitors changes over time when bias voltage is applied, however, which is a problem. This has led to consideration of using variable capacitance devices that utilize a plurality of variable capacitance elements that are connected in series and that include a dielectric layer formed via a thin film instead of by sintering.
When creating such a variable capacitance device, it is preferable that variable capacitance elements and resistors be formed upon the same substrate. As part of this process, an insulating moisture-resistant film and a conductive adhesive film are formed on the variable capacitance element portion of the device in order to keep the device moisture resistant. There have been mechanical reliability issues and current leakage problems in such devices, however, depending on certain factors. These factors include the relationship between the insulating moisture-resistant film and an interlayer insulation layer formed on the insulating moisture-resistant film, as well as the properties of the conductive adhesive film.